Psychology has ascertained that sight alone gives us no accurate sense
of the third dimension. In our infancy, long before we are conscious of
the process, the sense of touch, helped on by muscular sensations of
movement, teaches us to appreciate depth, the third dimension, both in
objects and in space.

In the same unconscious years we learn to make of touch, of the third
dimension, the test of reality. The child is still dimly aware of the
intimate connection between touch and the third dimension. He cannot
persuade himself of the unreality of Looking-Glass Land until he has
touched the back of the mirror. Later, we entirely forget the
connection, although it remains true, that every time our eyes recognise
reality, we are, as a matter of fact, giving tactile values to retinal
impressions.

Now, painting is an art which aims at giving an abiding impression of
artistic reality with only two dimensions. The painter must, therefore,
do consciously what we all do unconsciously,--construct his third
dimension. And he can accomplish his task only as we accomplish ours, by
giving tactile values to retinal impressions. His first business,
therefore, is to rouse the tactile sense, for I must have the illusion
of being able to touch a figure, I must have the illusion of varying
muscular sensations inside my palm and fingers corresponding to the
various projections of this figure, before I shall take it for granted
as real, and let it affect me lastingly.

It follows that the essential in the art of painting--as distinguished
from the art of colouring, I beg the reader to observe--is somehow to
stimulate our consciousness of tactile values, so that the picture shall